在swift的官方文档中，关于结构体closure给出这样一段描述：

Functions are actually a special case of closures: blocks of code that can be called later. The code in a closure has access to things like variables and functions that were available in the scope where the closure was created, even if the closure is in a different scope when it is executed—you saw an example of this already with nested functions. You can write a closure without a name by surrounding code with braces ({}). Use in to separate the arguments and return type from the body.

翻译整理一下：

1.函数其实是一种特殊的机构体：一段可以在之后相应的代码块。

2.结构体创建的时候参数可以是值或者函数，甚至是另一个结构体。

3.给出了结构体的创建方法，既用大括号{}覆盖，然后用in关键字将参数和返回值分开。

下面是官方文档给出的例子：